Great efforts have been made to make improvements in the design of robotic grippers; however, end effectors, or hands, for robotic arms continue to be a limiting element for efficient use of the robotic grippers. Frequently, robot grippers for space use employ finger elements containing "V" grooves to grip an object. These "V" grooves are used to both guide the object into place within the robot finger mechanism and to grasp and lock the object in that position once the object is securely in place. The "V" groove grippers have a frictional component at the interface between the object being grasped and the gripper finger. This frictional component causes several problems in the grasping process itself. First, it causes wear between the gripper finger and the object being grasped which, in its turn, rapidly leads to "burring" and contamination on both members. This frictional component also increases the force which the gripper motor must employ to pull the object in and seat it. This increase in the required force also translates into increased motor power and thermal control problems for the gripper motor. And, it reduces the effectiveness of the grooves in the robot finger to perform as a guide for aligning the grasped object with the robot fingers. Also, the frictional component confuses the force sensors associated with the gripper. The sensors are really detecting the vectorial sum of the grasping force plus the friction. But, friction is very unpredictable, thus sensor measurement accuracy is adversely affected. Also, as the object is pulled into the grasp of the fingers, the frictional component changes sporadically between static and dynamic friction causing a skipping effect which further confuses the sensors. Finally, the friction present when "V" groove fingers grasp an object can make it difficult for the gripper to release that object. This problem appears when the robot does not completely release the side loads and torques on the gripper before attempting to open the gripper jaws.
An alternate form of the "V" groove scheme is the pins and cones method wherein the cone acts as a multiple "V" groove, centering the object in the x, y, and z-axes; and allowing rotation about the x-axis and rotation about the y-axis. The pin resolves the rotation about the z-axis.
For large space robots such as those used on the Space Shuttle (for example the Shuttle Remote Manipulator System (RMS)), a snare technique is used to capture an object. The snare itself is comprised of two or more wire rods with hooked or curved ends mounted to a common housing. To capture an object or payload, the wire rods are rotated toward one another such that the edge of the rods having the hooked or curved ends engage the payload and pull the payload into the housing and hold the payload securely in position.
The snare system used in the RMS has disadvantages that differ from those of the "V" groove fingers. The snare system was primarily designed to capture spinning satellites in space; it has no frictional component when it grasps an object in space. However, the snaring process is a complex one because 1) the end effector requires a relatively long axial length to have enough range to search out and locate an object; 2) the several motors and gears located within the end effector are difficult to control such that an operator can manipulate the gripper with the precision movement necessary to quickly and efficiently grasp an object; 3) the end effector must also employ separate latches to secure the object once it is pulled in by the snare.
The apparatus disclosed in the instant invention overcomes the shortcomings of the "V" groove and snare grippers by providing an end effector gripper that will, with very low frictional forces, effectively guide, align and seat an object in a desired position without causing "burrs" or contamination that will damage the object.